In the present study a simple, fast, sensitive and robust method to quantify mirtazapine in human plasma using quetiapine as the internal standard (IS) is described. The analyte and the IS were extracted from human plasma by a simple protein precipitation with methanol and were analyzed by high-performance liquid chromatography coupled to an electrospray tandem triple quadrupole mass spectrometer (HPLC-ESI-MS/MS). Chromatography was performed isocratically on a C18, 5?mu m analytical column and the run time was 1.8?min. The lower limit of quantitation was 0.5?ng/mL and a linear calibration curve over the range 0.5150?ng/mL was obtained, showing acceptable accuracy and precision. This analytical method was applied in a relative bioavailability study in order to compare a test mirtazapine 30?mg single-dose formulation vs a reference formulation in 31 volunteers of both sexes. The study was conducted in an open randomized two-period crossover design and with a 14?day washout period. Since the 90% confidence interval for Cmax, AUClast and AUC0inf were within the 80125% interval proposed by the Food and Drug Administration and ANVISA (Brazilian Health Surveillance Agency), it was concluded that mirtazapine 30?mg/dose is bioequivalent to the reference formulation, according to both the rate and extent of absorption. Copyright (c) 2012 John Wiley & Sons, Ltd.